1. Field of the Invention
This invention relates generally to photovoltaic solar cells and to a method of making the cells. More particularly, the invention relates to the preparation and attachment of the top grid electrode in a thin-film photovoltaic cell.
2. The Prior Art
Cadmium sulfide solar cells operate on the principle of converting light energy to electrical energy. These cells comprise a thin film of cadmium sulfide deposited on a bottom electrode. On one surface of the cadmium sulfide a film of copper sulfide is placed which forms a barrier junction between the films. Without going into an explanation of how it is generated, the incidence of light on the copper sulfide layer, or as some maintain, on the cadmium sulfide, effects a voltage between the cadmium sulfide layer and the copper sulfide layer. In order to collect this current from the cell, a "collector grid" is applied to the copper sulfide layer to form the top electrode. By connecting appropriate wires to the top and bottom electrodes, a circuit can be formed for the flow of electrical current.
In the early development of solar cells, a collector grid was applied to the copper sulfide layer of a cadmium sulfide solar cell by drawing stripes with a ruling pen using a silver paste as the ink. However, poor adhesion, wide lines, and irregularities in width of lines presented problems in light transmission and electrical conductivity. Improved collector grids are disclosed in U.S. Pat. Nos. 3,442,007; 3,888,697 and 3,978,333. However, these grids also suffer from a number of disadvantages. The process of U.S. Pat. No. 3,442,007 requires the application of relatively high temperatures and pressures in order to get proper adherence of the grid to the cuprous sulfide layer. When an adhesive is used to attach the grid, such as in U.S. Pat. No. 3,888,697, the adhesive defeats the purpose of the top collector grid in that it insulates the collector grid and interferes with the conduction of current from the barrier to the grid. An approach used by the Clevite Corporation was to dip a preformed grid in a conductive epoxy-gold adhesive mixture prior to attachment to the cell. This approach presented numerous fabrication problems since it was very difficult to handle thin preformed grids. Another problem with using typical adhesives is that they must be applied as a liquid and therefore present various fabrication problems. What is needed is a collector grid that can be easily formed and attached to the top layer of the solar cell without the necessity for using additional adhesives and without the need for applying excessive heat and pressure. It is also necessary that the grid be in good ohmic contact with the top layer.